1. Field of the Invention
The invention relates to mobile communications networks, cyclic prefixes and a method for setting a mobile node specific cyclic prefix in a mobile communication system.
2. Description of the Related Art
A radio signal may propagate through different paths between a transmitter and a receiver. This is due to the fact that, for example, high-rise buildings and mountains reflect radio waves. A reflection may be further reflected. The reflections give rise to the delay spread of a signal. Multiple signals generated by a single transmitted signal may reach the receiver. The reflections are delayed and attenuated to some degree. The delay is due to the different lengths of the paths that the reflections may travel. The reflected signals that are delayed causes either gains in the signal strength or deep fades. In deep fades the signal level is so low that it is not possible for the receiver to determine the transmitted signal. Delay spread of signal reflections exceeding the duration of one Orthogonal Frequency Division Multiplexing (OFDM) symbol cause frequency-selective fading. The delay spread where the duration of one symbol is not exceeded is called flat fading. In order to mitigate the delay spreading, cyclic prefixes are used in the OFDM technology, which is a multicarrier modulation technology that uses orthogonal subcarriers to convey information. A Cyclic Prefix (CP) is a copy of the last part of a composite signal that is added to the front of the composite signal formed as an inverse discrete Fourier transformation of the modulated subcarriers. The cyclic prefix acts as a guard time between successive symbols. If the duration of the transmission of the cyclic prefix exceeds the maximum delay spread, the Inter-Symbol Interference (ISI) is eliminated.
In the Long-Term Evolution (LTE) system specified by the 3G Partnership Project (3GPP) that uses OFDM for downlink transmission, there is defined the use of two cyclic prefix lengths, namely a normal CP and an extended CP. In propagation environments where there is a possibility for multiple reflections having a long propagation time between the transmitter and the receiver, such as in a mountainous or an urban area, where the root means squared delay spread may exceed 4.7 μs, an Evolved Node B (eNB) needs to be configured to use the extended CP instead of the normal CP. The extended CP can handle up to 16.67 μs root mean squared delay spread in the received signal so that there is no signal deterioration. In severe cases, the eNB configures mobile nodes in the area of the eNB to use lower Modulation and Coding Scheme (MCS) to be less susceptible to radio path degradation. The disadvantage in the use of the extended CP is the smaller space for packet traffic payload, because in each 1 ms subframe there are only 12 symbols instead of the 14 symbols in the case of normal CP. As the consequence, the normal CP allows 17% higher bit rate compared with the extended CP, provided that the MCS and retransmission rate are comparable. In the area of a single cell there may be different propagation environments.
Delay spread may also be caused by the use of a spatially dispersed antenna system to communicate with a mobile node. In distributed base stations or eNBs Remote Radio Heads (RRH) are separate from a base station server. An RHH comprises the base station's RF circuits, analog-to-digital/digital-to-analog converters and up/down converters. The RRH also has operation and management processing capabilities and a standardized optical interface to connect to the base station server. RRHs may also be used in the case of Multiple-Input and Multiple-Output (MIMO) transmission between a base station and a mobile node. In the case of a set of RHHs in a cell to communicate with a mobile node, the mobile node may experience different delay spread depending on the position of the mobile node relative to the RHHs in the cell footprint. Therefore, there may be cases where in the area of a single cell a mobile node could use the normal CP instead of an extended CP.
In prior art the use of either an extended CP or a normal CP is configured on cell basis. The CP is configured based on delay spread measurements executed beforehand in the radio network planning phase. The use of either normal CP or extended CP is indicated to a mobile node in cell broadcast information or in handover signaling. Whether normal CP or extended CP is used in downlink direction is determined by a mobile node implicitly as the relative distance between Primary Synchronization Signals (PSS).
Due to the aforementioned factors it would be beneficial to be able to use different cyclic prefixes depending on the actual delay spread. The delay spread may be caused by different propagation environments or by a different position of a mobile node relative to a number of remote radio heads, antennas or antenna ports. The use of a cyclic prefix shorter than the extended cyclic prefix in the cases where the propagation environment or the position of the mobile node permits it would free radio resources for data transmission.
US 2010/0118806 discloses use of different cyclic prefix lengths depending on packet class, but it does not discuss the situation of different actual delay spreads within a single cell and the possibility of mobile node specific cyclic prefixes within a single cell.